Ecological Entomology (2001) 26, 16±24 The pattern, rate, and range of within-patch movement of a stem-galling ¯y 1 2 2 J. T. CRONIN, K. HYLAND andW. G. ABRAHAMSON 1Department of Biology, University ofNorth Dakota and 2Department ofBiology, Bucknell University, U.S.A. Abstract. 1. Information on the movement of insects is critical to understanding the spatial spread, dynamics, and genetic structure oftheir populations, as well as their interactions with other species. With this in mind, the movement behaviour ofthe stem-galling ¯y Eurosta solidaginis Fitch (Diptera: Tephritidae) was investigated. 2. Fluorescent-marked adults were released at a single location within pure patches ofthe host plant, tall goldenrod Solidago altissima, and their distributions censused repeatedly throughout the day. 3. Following their release, male and female ¯ies redistributed themselves in a manner that was well described by a simple-diffusion model. The diffusion rate was independent of¯y density and time since ¯ies were released. 4. Female ¯ies dispersed at a signi®cantly faster rate, and therefore farther on average, than males. Based on the diffusion model, it was estimated that at 2.5±3.0 h post release, males and females had a median dispersal distance of only 2.0 and 2.5 m respectively. Furthermore, 95% ofthe males were estimated to have dispersed no more than 5.9 m, and females no more than 6.4 m. 5. Post-release censuses suggested that ¯ies were most active during mid morning, disappeared from the site at a rate of 10±15% per hour (most likely due to mortality), and survived for less than 2 days. Based on the rate of spread, diel activity, and liberal estimates oflongevity in the ®eld, 50% ofthe ovipositing females were predicted to have had a maximum lifetime range of movement within a patch of hosts of < 51 m (95% were expected to have been limited to < 130 m). 6. These data are used to assess whether the absence ofa positive correlation between host-plant preference and offspring performance in this system could be due to the limited scale ofdispersal ofthis species relative to the spatial scale at which its oviposition behaviour has been studied. Key words. Diffusion model, Eurosta solidaginis, gall insect, goldenrod, mark± release experiment, movement, oviposition preference, preference±performance correlation, Tephritidae. Introduction recent reviews see Hanski & Gilpin, 1997; Tilman & Kareiva, 1997; Turchin, 1998; Bohonak, 1999). Quantitative Research over the past few decades has brought into clearer data on movement have also proved valuable in developing focus the role of movement in the evolution and ecology of management practices for pest species (Kennedy & Way, insect populations. The pattern, rate, and range ofmove- 1979; Stinner et al., 1983; Cronin et al., 1999). ment are particularly important parameters because they can Empirical studies ofherbivore host-plant choice may also affect the spatial spread, dynamics, and genetic structure of bene®t from a knowledge of insect movement. A majority of populations, as well as interactions with other species (for studies on the subject has failed to support the expectation from simple evolutionary theory that natural selection should favour Correspondence: J. T. Cronin, Department ofBiology, University of a positive correlation between host-plant preference and North Dakota, Grand Forks, ND 58202, U.S.A. E-mail: james_cronin offspring performance (e.g. Karban & Courtney, 1987; @und.edu Courtney & Kibota, 1990; Fox, 1993; Larsson et al., 1995; 16 # 2001 Blackwell Science Ltd Stem-galler movement patterns 17 Craig et al., 1999). The absence ofa correlation in ®eld as discrete patches that are easy to distinguish (McCrea & experiments may be a consequence ofa limited range of Abrahamson, 1987; Anderson et al., 1989). As ®elds mature, dispersal in the herbivore. In particular, ifthe herbivore's range however, clones introgress to form high density monospeci®c, ofmovement is small relative to the size ofthe habitat under but clonally diverse, patches ofgoldenrod (Abrahamson & study, only a fraction of the individuals will encounter, and have Weis, 1997). The existence ofextensive monospeci®c patches the opportunity to choose, the most suitable host plants within appears to be a relatively recent development in the history of that habitat. Thus, a positive correlation between host tall goldenrod in North America. Prior to the immigration and preference and herbivore performance may be obscured spread ofEuropeans across the continent, tall goldenrod was because ofthe incorrect choice ofspatial scale. As studies of probably much more sparsely distributed, primarily inhabiting herbivore host-plant choice and offspring performance are river banks and naturally disturbed areas (Marks, 1983). Thus, taken outdoors (e.g. Anderson et al., 1989; Craig et al., 1999), the stem gallers have had a relatively short period oftime to the scale ofherbivore movement will probably be ofgreater adapt to the current distribution oftheir host plant. signi®cance. To date, this has not been considered formally in In the north-eastern United States, E. solidaginis adults most ®eld studies of preference and performance. emerge from within goldenrod patches in mid to late May Researchers have focused on the movement of organisms (Uhler, 1951; Abrahamson & Weis, 1997). Males emerge ®rst both among spatially discrete patches and within patches that and await the arrival offemaleson the terminal buds oftheir are relatively homogeneous (for recent reviews, see Kareiva, host plant (Uhler, 1951; Craig et al., 1993). Immediately 1990; Hanski & Gilpin, 1997; Tilman & Kareiva, 1997; following mating, females oviposit preferentially into the buds Turchin, 1998). With regard to within-patch movements, which ofgenetically susceptible, tall, and/or herbivore-freeramets were the focus of this work, patterns of movement have often (reviewed by Abrahamson & Weis, 1997; see also Horner & been found to be relatively simple: the redistribution of many Abrahamson, 1999; Craig et al., 1999, 2000; Cronin & insects is well described by a simple model of random diffusion Abrahamson, 1999). Three weeks following oviposition, the (Kareiva, 1983, 1990; Turchin, 1998); however not all insect stem tissue shows visible signs ofswelling and by mid July the species exhibit within-patch movement patterns that can be galls reach full size and are spheroid in shape (Uhler, 1951; modelled as such simple processes (see Kareiva, 1983; Turchin, Weis & Abrahamson, 1986; Abrahamson & Weis, 1997). The 1998). For example, the redistributional patterns ofsome insects larvae overwinter in the third stadium within the galls, then are better described by more complicated diffusion models such pupate and eclose the following spring. Adults do not feed and as those that allow for heterogeneous rates of dispersal within can live approximately 7±10 days under optimal conditions in the population (Inoue, 1978; Okubo, 1980; Plant & the laboratory (Uhler, 1951; Hess et al., 1996; Abrahamson & Cunningham, 1991; Turchin, 1998; Cronin et al., 2000). Weis, 1997). Clearly, the patterns ofwithin-patch movement cannot be assumed and must be ascertained experimentally. In the work reported here, a mark±release experiment was Eurosta solidaginis movement used to quantify the pattern, rate, and range of movement of the stem-galling ¯y Eurosta solidaginis Fitch (Diptera: Experimental design. The pattern and range ofwithin-patch Tephritidae) within large homogeneous patches ofits host, movement of E. solidaginis was determined by marking adult tall goldenrod Solidago altissima (Compositae). Through ¯ies with ¯uorescent powder, releasing them within the repeated, non-destructive, censuses ofstem-galler distributions centres ofmonospeci®c patches dominated by goldenrod, over time, this approach had the advantage ofalso being and resighting marked individuals following dispersal. Adult capable ofestimating the loss rate ofindividuals during the ¯ies were obtained from galls collected during the winter from course ofthe study (through death or disappearance). The ®elds near Bucknell University, Pennsylvania (40° 57¢ 5.3¢¢ N, within-patch redistribution patterns ofmale and female¯ies 76° 51¢ 53¢¢ W). Galls were stored at ±10 °C to maintain insects were tested against the prediction ofa simple model ofrandom in diapause. To break diapause, galls were transferred to small diffusion. In addition, the diel activities, dispersal rates, loss screen cages placed within an environmental chamber at a rates, and lifetimerange ofmovement ofmale and female¯ies constant 26 °C, 85% RH, LD 14 : 10 h photoperiod. Emerging were quanti®ed. This work was motivated, in part, by a adults were removed daily and stored in a separate growth longstanding interest in the possible constraints in¯uencing chamber set at 10 °C (all other conditions identical). For this host-plant choice (reviewed by Abrahamson & Weis, 1997; see experiment, adult ¯ies that had been in the cold growth also Craig et al., 1999; Cronin & Abrahamson, 1999). chamber for 3±5 days were used. Flies experiencing this developmental regime show no detectable difference in oviposition behaviour from ¯ies reared under natural condi- Methods tions (Abrahamson & Weis, 1997; Cronin & Abrahamson, 1999). Natural history In the evening prior to a mark±release trial, the thorax and abdomen of E. solidaginis adults were dusted lightly with Tall goldenrod is a long-lived perennial that propagates ¯uorescent pigment (Dayglo Color Corporation, Cleveland,